Traditionally, plastics have been colored by tumble blending the plastic with a solid colorant or additive concentrate such as a coloring powder. The blended material is then fed into an extruder or molding machine where the resin and the colorant are plasticized and the colorant is incorporated into the molten plastic.
However, this method has a number of drawbacks. In many cases, a solid colorant or additive concentrate does not uniformly mix with plastic, or it does not disperse uniformly during molding. Further, solid colorants often segregate from the plastic during pre-molding handling because of particle size or specific gravity differences.
In recent years, liquid color concentrates have been developed in an effort to overcome the disadvantages of the traditional solid colorants. Liquid color concentrates are dispersions of a colorant in a carrier vehicle. Being liquids, these colorant concentrates may be accurately metered directly into molding machines or extruders by in-line metering systems, which help overcome many of the disadvantages of solid colorants. They have been used to color thermoplastic resins such as polypropylene, acrylonitrile-butadiene-styrene (ABS), polystyrene, poly(ethylene terephthalate) (PET) and the like. However, liquid color concentrates sometimes lack uniformity and tend to settle or separate into layers after standing at ambient storage conditions.
Several prior art techniques have been developed in order to achieve uniform and storage stable liquid colorant concentrates. One prior art technique requires that vehicles for liquid color concentrates be primarily based on expensive surfactants and wetting agents. For example, U.S Pat. Nos. 3,956,008 and 4,167,503 disclose colorants having surfactant based vehicles that provide an excellent pigment dispersion media and allow relatively high pigment loadings, at viscosities suitable for use in standard in-line metering equipment.
Although surfactant-based vehicles generally perform satisfactorily, they are subject to several drawbacks. First, the surface active agents used as dispersants are expensive, which can significantly add to the cost of coloring or modifying thermoplastic resins. Secondly, since the surfactants used in the vehicles are polar in nature and hygroscopic, the liquid color concentrates are often incompatible with certain plastics. This incompatibility can cause poor mixing in the molder or extruder, resulting in non-uniform color, such as specks of colorant or streaking of colorant. Thirdly, surfactant-based vehicles are often responsible for a decrease in output due to lubrication of the forming equipment, especially at high colorant loadings. For example, extrusion of PET is commonly adversely affected by standard liquid colorant concentrates through "screw slippage" in the molding machine or extruder. A further problem encountered with surfactant-based vehicles occurs in the extrusion of mono-axially oriented polypropylene. A phenomenon known as "water carry-over" occurs when incompatible surfactants move to the surface of the plastic and, being hydrophilic, tend to hold water. This results in an unacceptable amount of water being carried over from the quenching water bath to the take-up equipment. Another problem known as exudation occurs when incompatible vehicles exude, or spew up, to the surface of the molded plastic. Finally, since the colorant and an inert filler such as fumed silica are generally used to control viscosity, this results in a concentrate that is thixotropic, i.e.--one which forms a gel that must be broken up before it can flow.
A second prior art technique developed to achieve uniform and storage stable colorant dispersions involves the use of vehicles not primarily based on surfactants. This technique focuses on thixotropy to maintain a stable dispersion of the colorant or additive. As noted above, a thixotropic concentrate is one which will not readily flow under gravity, but which thins to a very low viscosity on stirring or other application of shear.
U.S. Pat. Nos. 4,341,565 and 4,444,714 disclose stable liquid gel colorants that comprise a suspension of organic pigments in a liquid vehicle that is a plasticizer and which is gelled by an organophilic, expanding lattice clay that is ion exchanged with alkyl ammonium ions. The gel is modified by the addition of certain hydroxyalkyl fatty amines or fatty imidazolinium quaternaries.
U.S. Pat. Nos. 4,571,416 and 4,624,983 disclose liquid color and additive concentrates that are dispersions of a colorant and/or other additive in a non-surfactant based vehicle. The vehicle includes a low molecular weight addition polymer and an essentially non-volatile solvent for the low molecular weight addition polymer. The vehicle may also contain a dispersion aid, such as oleic acid, especially when the colorant is an inorganic pigment.
However, the non-surfactant based, thixotropic colorant concentrates are also subject to certain limitations. For example, the thixotropic concentrates, because of their initially high viscosity, may not flow evenly in a vessel to the pump inlet and are therefore often not readily pumpable for metering a given amount of the colorant concentrate into an extruder or molding machine.
Thus, liquid color concentrates available to plastic processors in the past have been inefficient, expensive, and not completely satisfactory from the standpoint of achieving color uniformity. Accordingly, there continues to be a need for liquid colorant/additive concentrates that are (1) cost effective; (2) compatible with a wide variety of commercial thermoplastic resins; (3) suitable for use in conventional in-line metering and blending equipment; and (4) possess good shelf stability and uniformity.
U.S. Pat. No. 4,188,236 is directed to the production of a non-dusting pigment composition by contacting an aqueous dispersion of an organic pigment with
(a) a solution or dispersion of an alkaline soluble rosin acid derivative; PA1 (b) a sorbitan ester; and PA1 (c) a water-insoluble organic carrier having a melting point below 100.degree. C. at a temperature above the melting point of both the sorbitan ester and the organic carrier. PA1 (I) a vehicle comprising PA1 (II) at least one colorant or additive.
U.S. Pat. No. 4,415,705 is directed to high-solids pigment concentrates containing a dispersing agent formed by reacting a poly(lower alkylene)imine with a polyester comprising the reaction product of hydroxystearic acid, or its oligomer, with tall oil rosin. The patent describes a black pigment concentrate that includes about 4.8 parts by weight of a maleic-modified pentaerythritol ester of rosin which is reported to have a Brookfield viscosity of 282 poise initially and 368 poise after three days aging. The concentrate is reported to flow readily by gravity and can be handled by pumps commonly used in the art.